A program control is known for controlling an openable roof mechanically connected to two rack cables driven by the output gear, the three stop positions of the motor being obtained by means of electrical switches controlled by a cam wheel. As this cam wheel must effect less than one revolution per shifting cycle, a reducing gear train (on the order of 1/20) is provided between the output gear and the cam wheel.
This device is heavy and requires many switches. Moreover, it lacks precision: this is constant throughout the cycle, but insufficient at certain points and useless in certain regions where nothing occurs. Thus, the lack of precision of the stop positions of the openable roof, above all its closed position, results in leakages, noise due to the turbulence of the air, and entry of rain water into the vehicle. For example, the uncertainty in the position of openable roofs equipped with known program controls is about 4 mm.